WDM (Wavelength Division Multiplexing) is a well established technology which is being used extensively in Optical Transport Networks. With this technology one may inject in one fiber many optical channels on different wavelengths (usually called optical channels or “colours”), by a combining/multiplexing operation performed by a MUX device. The whole group of channels/wavelengths on the same fiber can be separated by a separating/demultiplexing operation performed by a DMUX device.
An OADM (Optical Add-Drop Multiplexer) may be schematically viewed (see FIG. 1) as an Optical Network Element composed of transport interfaces, referred to as Network Node Interfaces (NNI) along with User Node Interfaces (UNI). An OADM provides functions that allow multiple optical channels received at its UNI to be multiplexed into its Outgoing NNI (the ADD function); and to demultiplex various optical channels for transmission at its UNI (the DROP function). The path from the incoming/input fiber to the output fiber, excluding the dropped and added wavelengths is called the “express path”.
In other words, the main function of an OADM is to add and/or drop any selected number of the incoming wavelengths. All the remaining wavelengths (if any) which are not dropped, have to pass through the OADM express path essentially intact.
A classical configuration of OADM (shown in FIG. 2) performs:    1) full demultiplexing of a combined optical signal composed from a plurality of optical wavelengths (channels) incoming through an input fiber, 2a) dropping some optical channels, 2b) adding some optical channels, and 3) full multiplexing the remaining and added optical channels into the optical signal to be forwarded to an output fiber. The classical OADMs are known for their high insertion loss, i.e., there is usually more than 90% power loss through such OADM (more than 10 dB power loss). Moreover, the classical configuration is quite expensive.
There are many applications, for example in so-called Metro Ring Networks where only a small number of the incoming wavelengths (up to 4—for users' services) have to be separated for the ADD/DROP functions in one NE, and where the majority of the incoming wavelengths has to pass intact through the NE. The classical OADM configuration is therefore an inefficient solution for such cases.
For those cases where only a few wavelengths have to be dropped/added per one NE, a Single wavelength OADM (SOADM) was commercially proposed. A Single OADM makes use of an individual filter that performs either the DROP and the ADD function to a specific wavelength. The insertion loss (I.L.) of a single add/drop filter like that in the SOADM is typically 1.5–2.0 dB. In case a number of wavelengths are to be added/dropped, there is an option to connect the individual filters in series. However, the series combination of a considerable number of the single filters also introduces high insertion loss to the express path (i.e., to the wavelengths passing through).
Due to a considerable insertion loss of a classical OADM separating a single wavelength from a plurality of wavelengths transmitted via an optical fiber, as well as drawbacks of a SOADM, one presently accepted method of allocating wavelengths in an optical transmission line is the method of grouped wavelength allocation. To keep the advantage of low I.L. for more than a single wavelength, a so-called Grouped OADM (GOADM) includes a wide band pass filter for the ADD or the DROP operation. Each of these filters takes care of more than one wavelength (typically 4 adjacent wavelengths in the ITU-T Grid). The ITU-T wavelength Grid defines optical channels selected by the international standard organization ITU-T for use in Optical Networks/OADM Rings.
To this end, the modern telecommunication industry presently uses Grouped OADMs equipped with so-called grouped filters (FIG. 4). As of today, the grouped filter is a wide-band filter intended for adding or dropping a band of adjacent wavelengths to be further processed in the OADM or by other filters. In other words, one can use a single filter in an optical transmission line to filter a required group of wavelengths (i.e., to use a so-called primary filter), and then use further filtering to separate each wavelength in the group (by using so-called secondary filters). However, there is a problem of so-called “missed” wavelengths on the groups' borders due to the “slopped” response of the wide-band filters used for extracting the required groups of wavelengths (FIG. 5). At least one wavelengths adjacent but not belonging to a group at each side thereof is usually lost for transmitting any information in the network where such a wide-band group filter is present (in any of its OADMs). In a typical example of an optical network having a grouped OADM, only 32 optical channels, out of 40 nominally available channels, can be used due to this effect. Therefore, the bandwidth efficiency of the network with such OADM is only 80%.
However, the advantage of using the group filters rather than a series of individual filters is their low insertion loss being typically of about 1.5 dB for the group, as opposed to about 1.5 dB per channel (!) insertion loss in an OADM comprising single filters in series.
It should be emphasized that both the insertion loss of the today's OADM and the bandwidth efficiency thereof are very important parameters for a network utilizing the OADM. Reduction of the insertion loss saves the number of optical amplifiers and hence the network cost. The bandwidth efficiency increases the optical network capacity.